The invention relates to a method for operating a drive train for a motor vehicle including a continuously variable transmission and a torque converter with a converter lock-up clutch which can be engaged by the operating pressure of the hydrodynamic torque converter.
A conventional torque converter on which the invention is based, having a converter lockup clutch which can be activated by means of an axial piston, is known, for example, from German patent DE 39 15 186 C1.
In the arrangement disclosed in this publication, the converter lockup clutch which acts between a pump wheel and a turbine wheel can be closed by means of a working pressure acting on a piston. It is, however, disadvantageous that in overrun conditions an increased internal pressure is produced in the torque converter, which also acts on the piston and counteracts the working pressure of the converter lockup clutch so that, in particular in a two-channel torque converter, the precise actuation of a desired working point of the converter lockup clutch is made more difficult. In the worst case, the converter lockup clutch can be closed only partially, or cannot be closed at all. A comparable problem is already discussed, without a specific proposal for a solution, in the document DE 101 47 207 A1.
Furthermore, U.S. Pat. No. 5,141,089 A discloses a method in which the overrun deactivation of the engine during the closing process of the converter lockup clutch is delayed in order to increase the engine speed. This takes place even if the turbine rotational speed is higher than the pump rotational speed, i.e. in overrun conditions.
The Japanese document JP 08 326891 A finally discloses a continuously variable transmission with a torque converter between the engine and transmission. In said document it is described that during the closing process of the converter lockup clutch the turbine rotational speed is changed by means of a change in the transmission ratio in such a way that the decrease in the converter slip is compensated for and the operational variable which is monitored in the process is the converter slip.
It is the principle object of the present invention to provide an improved method with which the converter lockup clutch can be reliably closed in accordance with a predefined setting even in overrun conditions.